1. Field of the Invention
The invention relates to a twin-shaft or multi-shaft extruder.
2. Description of Related Art
Twin-shaft or multi-shaft extruders having shafts which are capable of being driven in the same direction substantially comprise closely intermeshing shaft conveyors as well as kneading blocks as conveying elements. For the purpose of easy adaption of geometry and material to the different process tasks, appropriate components in the required variety are usually formed in such a way that they are pluggable onto the supporting shaft so as to rotate therewith.
In order to align the required flexibility with the high demands on efficiency and safety, geometric limitations are indispensable, such as only one or few element lengths and/or the same mounting position at the ends, whereby the necessary screw pitches, element types and/or overall lengths are substantially restricted. In order to achieve the required adaptability of the elements, only the end positions and/or lengths have to be observed so that the geometric conditions in between, such as the pitch and the offset angle, continuous or in steps, can freely be selected from a procedural point of view. In the product throughput, effectiveness and the kind of energy transfer from the conveyor structures to the product affect both the temperature and quality of the product. In order to wet solids in powder form with viscous material and disperse and/or disaggregate them therein, shear flows or, better still, elongational flows have to be generated in the highly viscous material.
EP 0 002 131 B1 discloses a self-cleaning type extruder including two shafts and a two-flight screw profile in which one comb of the screw has a small distance and the second one a great distance from the inner wall of the housing.
As a result, two flights separated by a comb at a great distance from the wall of the housing are formed between two adjacent combs at a small distance from the inner wall of the housing. Thus, the product may freely flow from one flight into the other one, whereby it flows into the first flight until the latter is filled, whereas the second flight is filled partly at most. Hence, the product is subjected to the necessary homogenous treatment only in the first flight so that the product is treated unequally, since the exchange of material from the first flight to the second flight is insignificant.
However, the homogenization and dispersion of the solids in a viscous matrix, which can be achieved by means of the known extruders, is still unsatisfactory. This particularly applies to solids with a particle size in the micrometer (μm) range.